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Re: square tubing loads
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- Subject: Re: square tubing loads
- From: Charley Hamilton <chamilto(--nospam--at)uci.edu>
- Date: Thu, 29 May 2003 15:14:26 -0700
Stephen -
I am an aspiring engineer, first year undergraduate,
> designing a table using 2" x 2" square tubing as legs. *woof* I assume that the 2" dimension is cosmetic, since it's almost certainly not going to be necessary for a table. > I would like to know how to determine the lightest carbon > steel material (thinnest wall) capable of supporting loads of > up to 300 lbs., 400 lbs... Remember that, despite the planned design loads, furniture often ends up taking all kinds of unplanned loading scenarios. One of my fondest memories from undergraduate structural analysis is the day we designed a desk chair: Professor: "What if the person leans back on one leg like this? What loads do you have to consider in that case?" [Chair (and professor) topple backwards onto the floor] Class: "Umm, impact loads?" Think about what people might end up doing with the table: changing lightbulbs, storing large piles of textbooks, dragging it back and forth across the floor (although using 2" HSS will tend to discourage this), etc. Thinking about these cases will help you better understand how much overstrength to provide. Also, don't forget self-weight if that table is to be fabricated from structural steel. The required wall thickness is driven by four potential "failure" modes: - axial compressive yielding (unlikely for a table) - axial shortening under load (probably negligible for 2" sq HSS) - wall buckling (likely negligible unless you can find much thinner walls than I see in the HSS specification) - member buckling (only really a concern for long members given the kinds of loads you describe later in the email)
Shall I refer to charts? Should I
> know a formula for calculating the load limit using material > thickness as the variable? where might I find formulas for different > types of material, for example round tubing, angle iron, square bar? I would be prone to simply calc it out using first principles. Depending on the course work you have already completed, I would say that an elementary mechanics of materials course should allow you to compute the axial stresses in the column members and bending stresses in the table-top members (assuming the table acts like a frame), which will effectively give the the required moment of inertia for the section. Remember to model the frame as having a pinned base (the bottoms of table legs are generally unlikely to develop substantial moment resistance). Also, mechanics of materials should have covered simple Euler buckling, which you can check to ensure that you have an adequate moment of inertia (again, highly unlikely to be a problem in this case) to prevent member buckling. For wall buckling, however, you probably need to do a little digging to get an understanding of how to do the calculations. Purely on a hunch, I'd say you could go as low at 1/16" wall thickness and still be relatively safe against wall buckling for 2" HSS. As far as tables go, the American Institute of Steel Construction (AISC) has the Manual of Steel Construction (MSC), which is intended to facilitate either allowable-stress or ultimate-strength design (depending on your edition) of steel structures. However, you need to understand the basic principles which the MSC employs before you can effectively use it. Theoretically you could use the MSC for this purpose, but it seems gross overkill for designing a table. Of course, an HSS 2.5 x 2.5 x 1/8 has an axial capacity (phi*Pn) of just under 6 kips for a 16 foot unbraced span according to LRFD 3rd Edn. MSC Table 4-6, and that's already under 4 plf of self-weight, so your troubles may be over, assuming the slenderness checks work out. Check out the LRFD specifications for HSS (hollow structural sections), which is available free for download from AISC's website. This will give you some understanding of what you're getting into if you run a code analysis of the HSS table legs. Similar information is available for angles, round HSS, pipe, etc. If you're looking for *very* thin wall tubes, you may ned to check AISI specifications for material properties and run first-principle calcs for the axial deformation, yielding, and global/local buckling issues based on specific geometric data from metal suppliers. > Thank you in advance. It is to my great advantage to be > allowed to listen in to the conversations of you all I was in your shoes not *that* long ago, and it's in many ways to our advantage to have young engineers such as yourself "eavesdrop" on the conversations here. The reality of practice generally provides a different viewpoint on engineering than one would obtain in the classroom, which often enhances one's understanding of engineering. Best of luck and have fun designing the table. Charley Hamilton -- Charles Hamilton, PhD EIT Faculty Fellow Department of Civil and Phone: 949.824.3752 Environmental Engineering FAX: 949.824.2117 University of California, Irvine Email: chamilto(--nospam--at)uci.edu ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp* * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to:
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- From: Stephen Koontz
- square tubing loads
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